Coaxial transmission line



Feb. 17, 1948. L. A. BONDON 2,436,284

/ COAXIAL TRANSMISSION LINE Filed Dec. 26, 1946 IETET-EJ POL YTETEHFL008057197 [/Vf Lewis A. Bondon Patented Feb. 17, 1948 UNITED STATESPATENT OFFICE COAXIAL TRANSMISSION LINE Lewis A. Bonclon, Arlington, N.J.

Application December 26, 1946, Serial No. 718,489 Claims. (01. lie-2e)This invention relates to coaxial transmission lines and has moreparticular reference to insulating assemblies for maintaining theconductors of such lines in concentric relation.

Conventional forms of coaxial transmission lines usually comprise atubular inner conductor disposed in a tubular outer conductor and spacedconcentrically therefrom by insulating spacing devices. One conventionaltype of spacing means comprises insulating rings or discs usually madeof ceramic material and mounted on the periphery of the inner conductorat intervals therealong. The rings or discs are usually secured on theinner conductor by one of the following means:

(a) Rolled annular beads or expanded shoulders formed on the innerconductor on both sides of the insulating ring; (1)) soldered collars 0nthe inner conductor on both sides of the insulating ring; (0) annularsprings compressed to fit in undercuts in the discs and inner conductor;((1) annular spring supported locking pins indexing in the innerconductor and with recesses in the discs or rings; and (e) friction.

Another conventional type of spacing means comprises radial insulatingrods or pins usually made of ceramic material and secured to the innerconductor at intervals therealong. The rods or pins are usually securedto the inner conductor by one of the following means:

(a) Pins secured by cement to the surface of the inner conductor; (b)pins secured by annular staking at inner conductor; (0) pins supportedby outer conductor and not attached to inner conductor; (d) pins securedby threaded inserts in inner conductor; (e) pins supported by supportsthrough their body, then staked to the inner conductor.

All of these assemblies, whether of the insulating ring type orinsulating pin type, are defic ent for one or more of following reasons:

a) Large proportions of solid dielectric; (b) sensitive to thermalshocks; (0) undesirable diamctric irregularities; (11) poor physicalproperties: (e) reduced amount of radio frequency conducting surface;(1) unpredictable dielectric sta bility; (g) diflicult to repair afterassembly; and (h) costly and difficult to manufacture.

The object of all the foregoing methods of coax al line construction isto obtain a signal propagating medium having a propagating constant thatis close to that provid d by air. This propagation constant is reducedas soon as dielectrics having specific inductive capacity values higherthan air are introduced into the propagating medium. Therefore thedesirable coaxial transmission line must have a minimum mass of lowdielectric body as supporting and spacing elements as will allow maximumphysical and thermal performance under various service conditions forlong periods of time without need for replacement or maintenance.

In addition. to the foregoing, the inner and outer conductors must hemaintained as accurately as possible in concentric relation throughoutthe transmission system in order to provide undistorted signalpropagation from the signal source to the terminating elements. Whenmomentary or sustained high voltage gradients exist along the line it isnecessary to have this accurate concentric relation to provide auniformly distributed electrical stress at the points of support toprevent localized dielectric heating and possible thermal failure and toprevent lower corona initiation voltages producing electrical flash-overat these points and a subsequent inoperative system. At these points ofsupport it is necessary that all air be excluded between the dielectricand the inner conductor, chiefly because the voltage gradient is maximumat these points and will often cause local ionization, thereby reducingthe corona initiation voltage of the tran mission line as well asintroducing distortional effects on the propagating signals.

It is an object of the present invention to provide a coaxialtransmission line in which the inner and outer conductors are held inaccurate concentric relationship, throughout their length, bylongitudinally spaced, angularly disposed insulating spacing assemblies,each of which comprises a pair of diametrically opposed insulatingspacing members positioned between the conductors and held in positionby an insulating pin passing through the inner conductor with its endsengaging the spacing members.

Another object of the invention is to provide a coaxial transmissionline, as characterized above, in which the pins and spacing members aremade of insulating material of such character and are assembled in suchmanner as to provide great resistance to thermal and physical shock.

Another object of the invention is to provide a coaxial transmissionline, as characterized above. in which the spacing members andsupporting pins are made of insulating materials which provide apropagating medium substantially the same as air, thus providing highersignal transmission efiiciency to permit its use at frequencies beyondthe range of the conventional air dielectric coaxial transmission line.

Another object of the invention is to provide a coaxial transmissionline, as characterized above, in which the supporting pins and thespacing members are made of insulating material of such character andare assembled in such manner as to obviate the necessity for the usuallinear dimensional tol rances, thereby providing a. differentialimpedance. limit heretofore unobtainable for commercial application.

A further object of the invention is the provision of a coaxialtransmission line that can be readily assembled and repaired in bothfactory and field and one that is relatively inexpensive tomanufacture.

Other objects and advantages will be apparent in the followingspecification, when considered together with the accompanying drawings,in

I which:

Fig. 1 is an elevational view with parts broken away of a coaxialtransmission line constructed in accordance with the present invention;

Fig. 2 is an end view with certain parts shown in section, of thecoaxial transmission line shown in Fig. 1;

Fig. 3 is an end view of one of the insulating spacers shown in Figs. 1and 2; and

Fig. 4 is an elevational view of the insulating spacer shown in Fig. 3.

7 Referring to Figs. 1 and 2, there is shown a. coaxial transmissionline comprising an outer conductor l and an inner conductor l l. Theconductors are held in concentric relationship, throughout their length,by means of longitudinally spaced, angularly disposed insulating spacingassemblies mounted on the inner conductor.

Each insulating assembly comprises a diametrically opposed pair ofspacing members or caps l2 interposed between the inner and outer conductors and engaging diametrically opposed sections on the inner surfaceof the outer conductor and held in position by means of an elongated pinl3 extending through diametrically opposed holes H-il formed in theinner conductor with its projecting ends |5l5 press-fitted into recessesiii-l6 formed in the caps l2.

Each of the caps I2 is identical in construction and, in the particularembodiment illustrated, is shown as a cylindrical member having acoaxial cylindrical recess l6 provided with a conically shaped bottom I!formed therein. One end of the cylinder is wedge-shaped, as indicated atl8, and the other end is made concave, as indicated at IS, with asurface curvature the same as the outer surface curvature of the innerconductor. The caps are preferably made of a compressible plasticorganic insulating material such as polytetrafiuoroethylene. This is anextremely lowloss material. It is physically capable of maintainingoperating efliciency at the elevated temperatures encountered in normalservice and is also physically capable of maintaining intimate contactwith the conducting surface of the inner conductor throughout theservice life of the line. Cap members made of this material will notcarbon-track on flash-over and do not require surface impregnatingtreatments to reduce its moisture absorption factor as the material isnormally nonhydroscopic.

Each of the pins I3 is identical in construction and in the particularembodiment illustrated is shown as an elongated cylindrical memberhaving conically shaped ends l5-l5. The pins are made of an organicinsulating material, The material preferred is a thermosetting styrenecopolymer such as ethyl vinyl benzine div nyl ben- 4 zene styrenecopolymer. Such a copolymer is disclosed in U. 8. Patent No. 2,256,160,issued Septempber 16, 1941. The pins may also be made of methylmethacrylate or of ceramic material composed principally of magnesiumoxide. Pins made ofthe foregoing named materials are not subject todeformation, cracking or plastic flow. They have great resistance tophysical shock and are physically capable of maintainin maximumoperative efliciency at the elevated temperatures encountered in normalservice exposures. The diameters of the pins are such that they fittightly in the holes i6, I4 formed in the inner conductor to eliminateany air therebetween.

In the preferred construction, the pins are approximately one-half thediameter of the caps and approximately one-fourth the diameter oi. theinner conductor.

In assembling the transmission line, the supporting pins i3 are fittedin the openings i4, H in the inner conductor. Then, the caps 12 withtheir wedge-shaped ends positioned longitudinally, are press-fitted ontothe projecting ends of the pins and against the outer surface of theinner conductor.

The concave end IQ of each cap provides an intimate physical air-tightsaddle fit with the outer surface of the inner conductor, as indicatedat 20. Then, the inner conductor, with the spacing assemblies in place,is press-fitted into the outer conductor, the plastic material of whichthe cups are made being slightly compressed to make an intimate physicallit with the inner surface of the outer conductor. To facilitate theinsertion of the cap member into the outer conductor, the longitudinalends of the wedge-shaped end l8 may be bevelled. as indicated at 2|.

The insulating assemblies are longitudinally spaced at desired intervalsand may be disposed on the inner conductor in any desired angularrelation. In the particular embodiment illustrated, they are shown asbeing angularly disposed relative to adjacent assemblies.

Spacing assemblies employing a combination of extremely low-lossdielectrics, as above set forth, and assembled in the manner described,substantially eliminate all air in the dielectric field at the points ofassembly and reduce the fiux density in the electric field at thesepoints, The elimination of air in the dielectric field at the points ofassembly minimizes the probability of any local corona. formation atthese points. The low-loss properties of the dielectric combination atany service frequency encountered in the application of an airdielectric transmission line affords stable dielectric performance,which, with the positive centering of the inner conductor, provides astable and continuously uniform propagating medium free from diametricdiscontinuities.

The spacing members or caps l2, being made of compressible plasticmaterial press-fitted between the inner and outer conductors, permit theuse of dimensional tolerances which cannot be effected with conventionalinorganic materials without a large increase in processing cost andsubsequent attendant assembly difficulties increasing the final cost.

A coaxial transmission line constructed in accordance with thisinvention, does not require the usual linear dimensional tolerances,thus providing a differential impedance limit heretofore unobtainablefor commercial application. This provision will permit a wider field ofapplication for coaxial transmission lines as such dimensionallimitations restrict the development of high outer conductor alongdiametrically opposed lonstanding waves to less than 10% of the voltagegitudinal sections thereof and wherein said elonstanding wave ratiointroduced at the signal gated insulating member is cylindrical and hassource. its ends contoured to conform to the contour of In addition,such a construction provides a the inner surfaces of said recesses.minimum signal attenuation for a transmission 3. A coaxial transmissionline, as set forth in line of a given impedance. This is due to theclaim 1, wherein said elongated member is made low-loss dielectriccombination, which will, for a oi a thermcsetting styrene copolymer.

given line impedance, permit the use or a larger 4. A coaxialtransmission line, as set forth in center conductor to provide anincrease in radio claim 1, wherein said elongated member is madefrequency conducting surface with attending inof ethyl vinyl benzinedivinyl benzine styrene cocrease in power handling capacity. polymer.

From the foregoing, it will be seen that there 5. A coaxial transmissionline comprising tubuhas been provided a coaxial transmission line larinner and outer conductors and spacing ashaving an impedance andproviding a propagatsemblies radially disposed within said outer coningmedium substantially the same as that of a ductor at predeterminedspaced points throughtheoretical coaxial transmission line of the sameout its length for maintaining said conductors in dimensions in whichthe inner conductor is supconcentric relation, each of said assembliescomported solely by air. prising a pair of diametrically opposedcylindrical While the invention is particularly described in 2 canmembers provided with coaxial recesses and connection with a coaxialtransmission line havp ess-fit ed b ween the inner surface of the ing atubular inner conductor, obviously, it is outer conductor and the outersurface of the equally well suited to a coaxial transmission line innerconductor, sa d p members being a e having a solid inner conductor. ofpolytetrafluoroethylene and having concave Obviously, the invention isnot restricted to the inner ends to provide intimate physical contactparticular modifications herein shown and dewith the outer surface ofthe inner conductor and scribed. wedge-shaped outer ends to provideintimate I claim: physical longitudinal line contact with the enter 1. Acoaxial transmission line comprising tubuconductor, and an elongatedcylindrical pin exlar inner and outer conductors and spacing astendingdiametrically through Said n r semblies radially. disposed within saidouter conductor and having its ends press-fitted into the ductor atpredetermined spaced points throughrecesses formed in said cap members,said pin out its length for maintaining said conductors in being made ofethyl vinyl benzene v ny benconcentric relation, each of said assembliescomzene styrene c D Y and having t 011- prising a pair of diametricallyopposed insulating g toured to conform to the contour of the innermembers press-fitted between the inner surface surfaces of saidrecesses.

of the outer conductor and the outer surface of LEWIS a nonnon. theinner conductor to provide intimate surface contacts between saidinsulating members and REFERENCES CITED said inner conductor, saidmembers 00 1 1 1 1 8 0f 40 The following references are of record in thepolytetrafluoroethylene having coaxial recesses m of this patent; formedtherein and an elongated insulating member extending diametricallythrough said inner UNITED STATES PATENTS conductor and having its endspress-fitted into Number Name Date the recesses formed in said firstnamed members. 46 2,216,893 Smith Oct. 8, 1940 2. A coaxial transmissionline, as set forth in 2,218,921 Mueller Oct. 22, 1940 claim 1, whereinsaid first named insulating mem- 2,256,160 Britton Sept. 16, 1941 bersare cylindrical and have concave inner ends 2,287,201 Scott et al June23, 1942 and wedge-shaped outer ends, said concave ends 2,397,568 SeamanApr. 2, 1948 having a curvature substantially the same as the curvatureof the outer surface of the inner con- OTHER REFERENCES ductor toprovide intimate physical contact there- Article entitled Design datafor beaded coaxial with, thereby excluding all air therebetween andlines," by Cox, published in Electronics Magazine said wedge-shapedouter ends provide intimate for May. 1946, pages -136. physicalcontactwith the inner surface of the $6

